Mike Leigh has a hit on his hands with Two Thousand Years, his first ever Jewish play. So why has he never spoken about his Jewishness until now? Perhaps, says Linda Grant, because it informs everything he does .

The question to ask Mike Leigh about his "Jewish play", Two Thousand Years, is surely not why a Jewish play, but why a Jewish play now? I last saw Leigh during the interval a few days after the play first opened at the Cottesloe Theatre last September, when audiences who had booked tickets for a "play by Mike Leigh", with no indication of its content, were absorbing the full impact of a stage-full of what I called "shouty Jews". He had given interviews back then, but refused to talk about the play itself, let alone why he had written it.

Mike Leigh's work, as filmmaker and playwright, has always seemed to be about Englishness, about the turmoil and pain that lies beneath the veneer of ordinary lives. His famous characters range from the vulgar, socially aspiring Abigail, of Abigail's Party, to Vera Drake, a 1950s back-street abortionist. Most famously, he works through improvisation, with his actors contributing to the dialogue and character development, though with key plot details withheld to reinforce the emotional weight of the actors' reactions. Although Leigh satirises the middle classes, his work is rarely didactic. It is hard to take a message away; there is no call to action, no larger political themes. Things erupt, then subside. Life goes on, only marginally altered. So here is a play that is one long row about the world's most intractable political conflict, Israel, opening with one of the main characters reading an article on the subject in the Guardian.

So why now? It seems to me that there could be two possible answers: either he had had some recent personal experience, perhaps saying Kaddish [mourning rituals] for a parent, that had brought to the surface some not yet articulated questions about his past, his Jewishness, his family; or he was responding to a particular political context that has emerged since the collapse of the Camp David negotiations in 2000, and the aftermath of 9/11 when Jews have found themselves thrust blinking back into the headlamps of political attention, under suspicion for any attachment to Israel and the subject of conspiracy theories.

His office, in a shabby Georgian house in Soho, London, is modest. You could film a domestic scene right there, apart from the framed pictures of his productions on the walls. Why now? "Why not, is the Jewish answer," he says. He then gives two disappointingly pragmatic reasons. First, Nicholas Hytner had commissioned him to write a play for the National Theatre, "and I went to the Cottesloe and saw Elmina's Kitchen with a completely black cast. Instinctively, I knew that whatever I chose to do would be about people discussing politics. The National is a forum for dealing with issues."

He says that a number of his previous plays had laid down the tracks for Two Thousand years, including Four Days in July, set in Belfast, and Greek Tragedy, about the Greek community in Australia: "Some of my friends who saw it asked me if it was a Jewish play in disguise."

These were, he says, the preparation "for doing yet another anthropological study". The difference was that this time he was examining his own community. He has, he explains, for many years been involved with the Afro-Asian Committee of Equity, which campaigned against white actors blacking up. "In an informal way I have always been a bit African-Asian Committee about non-Jews playing Jews, so part of the agenda was about casting Jewish actors. What we haven't done is wheel in the self-appointed kings and queens of Jewish acting. On another level, it would be totally inaccurate to suggest that this is my first Jewish play. I don't think you can pull out any play or film from my canon that is not Jewish in its view of life and all its tragi-comic aspects.'

I'm not sure how satisfying these answers are, but whatever Leigh's motives for writing the play, the fact remains that you have this big fat Jewish play commanding huge audiences at the National Theatre - it transferred to the larger Lyttelton two weeks ago. And it evidently comes directly out of Leigh's personal experience of growing up Jewish in Manchester, though it is set in the north London suburbs. The play is about a vast family argument. That argument is about love or lack of it, loneliness, sibling rivalry, discontentment with life, but part of that discontent is the family's disappointment at what the early idealism of Zionism has dwindled into, for if there is a son who inexplicably turns religious in a secular household, as the grandfather, Dave, points out, at least he believes in something. These people are believers, godless, middle-of-the-road socialists, let down after the long wait for a Labour government. Yet hope for a better future is what holds them all together. "The received wisdom," says Leigh, "is that modern political Zionism came into being in the late 19th century, but that dream to achieve something important is inherent in the Jewish psyche. Next year in Jerusalem."

Leigh's parents met in 1936 at Habonim, the Jewish socialist Zionist youth group in Manchester. Both his mother's sisters made aliyah (emigrated to Israel), one in 1934, another in 1949. His parents had also considered making aliyah themselves but married at the beginning of the war and by the time it ended, his father had a doctor's practice in Salford. "There were always people in and out of our house who were off on aliyah," Leigh says "and I'm just old enough to remember everyone hunched around the radio listening to Kol Zion La Golah" [The Voice of Zion to the Diaspora].

It was a kosher home, though they only went to synagogue now and again and they drove on the sabbath. "But it was very, very Jewish. My grandparents were immigrants - they talked in Yiddish - and there were some outreaches of the family where there were genuine frummers [Orthodox]."

It was a time, he points out, when many Jewish families were anti-Zionist, and there was a wave of post-war anti-semitism in the north of England. After the Zionist militant group, the Irgun, kidnapped and murdered two British sergeants in Palestine, there were anti-semitic riots and Jewish shop windows were smashed in Manchester and Liverpool. But in 1960, like many Jewish teenagers, he went with Habonim to Israel for the summer, and hitchhiked there on his own the following year. Nevertheless, when he became a student at the Royal Academy of Dramatic Arts in London, he consciously escaped his Jewish identity because, as he writes in an introduction to the play, he was escaping being stereotyped; having so many other possibilities closed down. This seems to be a peculiarly British dilemma that does not afflict American Jews; in the US almost everyone comes from somewhere else. To be British is to have to suppress where you came from, to pretend you always came from here.

"In many ways, my parents in an unconscious way wanted to be as English as possible," he says. "They went to the theatre, to Stratford, to the Halle. My father only ever voted Labour. They were very bourgeois, very neurotic and very insular. When I went to Israel in 1960 we were on a kibbutz and we insisted on having a discussion about whether you could be an artist on a kibbutz. Of course, what we were dealing with was our own, unformed struggle about the whole thing, where our cultural roots were. The truth was, we were European and English artists."

But as soon as this most English of playwrights and film-makers starts talking about being Jewish, you understand how it is so central to who he is that it hardly needs any external expression. He hasn't needed to write about Jews because Jewishness informs everything he writes anyway. "We had a discussion last night in the green room about whether converts were really Jewish," he says. "I'm more comfortable with people having a sex change than converting to Judaism because at least people who have a sex change are motivated from within." So what is being Jewish? In the play, the grandfather, Dave, answers this question with the observation that, "It means wanting to spend a couple of hours with your family on a Saturday afternoon and walking into a fucking war zone."

"A Jew is someone who grows up in that environment," Leigh says. "You can say that you're working class, you can say that you're northern or southern. These are non-negotiable facts. While I walked away from a Jewish existence, lots of things carried on in my life: gastronomic obsession, massive amounts of reading Isaac Bashevis Singer and Saul Bellow. So one doesn't stop being Jewish."

Leigh did not return to Israel until 1991, and only because his then wife, the actor Alison Steadman, was invited to the Jerusalem Film Festival. He didn't want to go, like many Jews disillusioned with what the country had become, but she talked him into it. He went to the Old City for the first time. "I haven't been since, and on the whole I'm not motivated to go," he says. He doesn't want to get into a discussion about Israel, Zionism or the conflict, though the play specifically discusses the Gaza disengagement.

Does he think there has been a rise in anti-semitism since the start of the intifada? "There is no doubt whatever that there is a context in which there is a confusion between Jews and Israel," he says. "We're all aware of that. That Israelis, for better or worse, bring it on themselves, is all I'll say. The play is a reflection, rather than a conclusive didact because that's what I do. It's a play that works as my plays work, which is not to be a mouthpiece for systematically ordered political argument. It's interesting to reflect on things, maybe in a piecemeal way."

The reception to the play has been positive. There have been no denunciations in the Jewish Chronicle, no pickets or accusations of his being a self-hating Jew, no pro-Palestinian protests either. Arnold Wesker apparently thought it should have been more argumentative. Packed out audiences at the Lyttleton indicate that it is finding a wide audience.

Watching the play as a Jew, I was aware that what was dramatised on stage was an argument that had been going on for some time inside my own head and among friends and family. It was as if one's own internal contradictions were being conveniently staged for you. "One of the challenges of the play was judging to what extent the general audience needed explanations," he says. "I tend to think that you've got to assume the audience is as intelligent as you are, and informed to a reasonable degree."

The production has toured Bath, Salford, Warwick, Malvern, Newcastle and Cambridge. In Bath, someone asked whether it was a play for a Jewish audience. 'The idea of putting on a play at the National Theatre which is just for Jews is preposterous. Vera Drake wasn't aimed exclusively at people who have had abortions. And in Manchester a man said, 'Is it not obvious that this play was set up to attack Bush and Blair?'"

In the play, the daughter, with her Israeli boyfriend, reflects how Zionism has changed from being "something positive and hopeful" to a dirty word. For the family, Zionism has been a disappointment because it has not lived up to its own founding values. To others, I point out to Leigh, it had only ever been a colonial movement bent on ethnic cleansing. "Given the events of even the 19th century, Zionism was inevitable," he replies. "Given the events of the 20th century, Israel was inevitable."

The play is about disappointment, he says, twice during the interview. Political disappointment and, perhaps, disappointment at how one child, of whom you had high hopes, has turned out.